The present invention relates to an improved noise limiting circuit for an FM stereo receiver operable to automatically suppress high frequency noise components and switch from a stereo to monaural mode in response to a weak electric reception field.
It is well known that high frequency noise becomes noticeable in the demodulated signal in an FM receiver when the received signal strength becomes weak. Accordingly, the S/N (Signal-to-Noise) ratio can be considerably improved by cutting off or squelching the high frequency components of the demodulated signal in response to the lowering of the received signal strength. The S/N ratio can be further improved by automatically switching from the stereophonic reception mode to the monaural reception mode in response to such signal weakening. Although receivers which implement such cutoff and changeover are well known, they involve frequent switching operations when the received signal strength fluctuates around the switching threshold level, and such a phenomenon is particularly noticeable in a moving car radio. Consequently, noise due to the switching operations is produced, which disrupts the quality of the sound reproduction.